Ch. 8 – Alkyl Halides & Elimination Reactions

Question 1

What is an elimination reaction?

Answer 1

loss of starting materials to form a pi bond

Question 2

What substance do alkyl halides undergo elimination reactions with?

Answer 2

bronsted-lowry bases

Question 3

What is dehydrohalogenation?

Answer 3

removal of H and X; beta elimination because it involves the loss of elements from two adjacent atoms

Question 4

What is the alpha carbon?

Answer 4

carbon bonded to X

Question 5

What is the beta carbon and can there be more than one?

Answer 5

carbon bonded to H right next to alpha carbon; yes

Question 6

What three steps occur for general elimination mechanism?

Answer 6

1) base removes proton from beta carbon and forms H-B+
2) electron pair in beta C-H forms new pi bond between alpha and beta
3) new pi bond boots off electron pair in C-X bond which goes with leaving group

Question 7

What are the four most common dehydrohalogenation bases?

Answer 7

(alkoxides)
Na+/ -OH sodium hydroxide
K+ / -OH potassium hydroxide
Na + / -OCH3 sodium methoxide
Na+ / -OCH2Ch3 sodium ethoxide
K+ / -O(CH3)3 potassium tert-butoxide

Question 8

T/F: Having more than one beta carbon will lead to multiple products that are constitutional isomers.

Answer 8

true

Question 9

What are alkenes?

Answer 9

products of elimination, C=C

Question 10

What are important factors about alkenes (geom, hybridization)?

Answer 10

sp2 hybridized, trigonal planar (120º)

Question 11

What does double bond consist of? What types of overlap cause these bond types?

Answer 11

one sigma and one pi bond

• sigma caused by end-end of sp2 overlap (lies in plane)
• pi caused by side-side of 2p overlap (lies perpendicular and has restricted rotation

Question 12

What are mono/di/tri/tetrasubstituted alkenes?

Answer 12

number of carbon atoms bonded to carbons of double bonds

Question 13

Does a double bond have free or restricted rotation? What does this cause?

Answer 13

restricted rotation, stereoisomers (cis and trans)

Question 14

When are diastereomers possible?

Answer 14

whenever two groups on each end of C=C double bond are different

Question 15

Are cis or trans alkenes more stable?

Answer 15

trans because groups are further apart and there are reduced steric interactions

Question 16

How does the number of R groups bonded to C=C affect stability?

Answer 16

more R groups = more stable

Question 17

Why do R groups increase stability?

Answer 17

they are sp3 hybridized and donate electron density to lower % s character sp2 carbon (inductive effect)

Question 18

Do sp2 or sp3 more readily accept electron density?

Answer 18

sp2

Question 19

What are the two types of elimination mechanisms?

Answer 19

E2 and E1

Question 20

What is an E2 mechanism?

Answer 20

one step (concerted), second order, bimolecular

Question 21

What is rate equation for E2 mech?

Answer 21

rate = k[R-X][base]

Question 22

What are general steps for E2 mechanism?

Answer 22

1) base removes proton from beta carbon
2) C-H bond breaks and electron pair forms pi bond
3) leaving group breaks off and takes second electron pair

Question 23

What are the characteristics of the energy diagram for E2 mech?

Answer 23

• one barrier (hump)
• transition state has 4 partial bonds
• entropy favors products (more molecules of product)
• products lower energy

Question 24

What factors affect rate of E2 mech?

Answer 24

alkyl halide: more substituted = faster rate (opposite of Sn2)
strong base = faster rxn
better leaving group = faster rxn
polar aprotic solvent = faster rxn

Question 25

T/F: When there are two or more beta carbons, equal amounts of product are made.

Answer 25

false, one product predominates

Question 26

What is the more major product in elimination reactions?

Answer 26

major product = more stable product (more subs on double bond)

Question 27

What is the Zaitsev rule?

Answer 27

major product in beta elimination has more substituted double bond

Question 28

What does regioselective mean?

Answer 28

when a reaction yields more of one constitutional isomer (E2 is regioselective)

Question 29

Why does Zaitsev rule occur?

Answer 29

double bond is partially formed in transition state so increasing stability of double bond lowers Es for reaction and rate increases

Question 30

What product is major when stereoisomers are formed?

Answer 30

more stable stereoisomer (trans)

Question 31

What does stereoselective mean?

Answer 31

when reaction produces more of one stereoisomer when two or more are possible

Question 32

What is E1 mechanim?

Answer 32

two steps, first order, unimolecular

Question 33

What is rate equation for E1 mechanism?

Answer 33

rate = k[alkyl halide]

Question 34

Which bond breaks first in E1 mech?

Answer 34

bond to leaving group breaks before pi bond is formed

Question 35

What are steps of E1 mech?

Answer 35

1) heterolysis of C-X bond forms carbocation (RDS) | 2) base removes proton from carbon adjacent to carbocation and electron pair in C-H bon forms new pi bond

Question 36

What are characteristics of energy diagram for E1?

Answer 36

- two humps | - step one is higher hump (RDS)

Question 37

How do elimination reactions produce alkynes?

Answer 37

two consecutive elimination reactions form two pi bons

Question 38

What are the two starting materials that can be used to form an alkyne?

Answer 38

vicinal dihalide or geminal dihalide

Question 39

What is a vicinal dihalide?

Answer 39

has two X atoms on adjacent carbon atoms

Question 40

What is a geminal dihalide?

Answer 40

has two X atoms on same carbon atoms

Question 41

What is the intermediate product formed when forming alkynes?

Answer 41

vinyl halide

Question 42

What type of bases are needed to form an alkyne?

Answer 42

VERY strong bases (usually amides -NH2) because it is harder to remove H and X from sp2 hybridized carbons

Question 43

T/F: Sn1 and E1 have exact same first step

Answer 43

true, both produce carbocation

Question 44

How do Sn1 and E1 differ in step two?

Answer 44

Sn1: nuc attacks carbocation to form substitution product E1: base removes a proton and forms pi bond

Question 45

T/F: the conditions for E1 to occur and the same as for Sn1 to occur

Answer 45

True (3º alkyl halide, weak nucleophile/base, polar protic solvent)

Question 46

What happens when E1 or Sn1 reaction could occur?

Answer 46

forms mixture

Question 47

How many atoms does the transition state of E2 reaction have in its plane?

Answer 47

4 (co planar C-x and C-H)

Question 48

What are the two conformations for the E2 transition state?

Answer 48

syn periplanar and anti periplanar

Question 49

What is syn and anti periplanar?

Answer 49

syn periplanar is when H and X are on the same side | anti periplanar is when H and X are on opposite sides

Question 50

What is needed to E2 reaction, syn or anti periplanar geometry?

Answer 50

anti periplanar because it is staggered and therefore lower in energy which allows electron rich base and leaving group to be further away from eachother

Question 51

For halocycloalkanes, how must geometry occur for E2 reaction to take place?

Answer 51

must be anti periplanar and have trans diaxial position for X and H

Question 52

What is trans diaxial geometry?

Answer 52

C-X and C-H are both in anxial position of ring

Question 53

Are both of the cis conformation rings reactive for E2? How many difference products can cis form?

Answer 53

no, only X axial conformation is reactive can form two different products because it has two beta hydrogens in axial position (minor or major depending on alkene substitution)

Question 54

Are both trans ring conformations reactive for E2? How many different products can trans ring form?

Answer 54

only trans with axial halogen is reactive can only form one product because there is only one axial beta hydrogen

Question 55

Should stereochemistry be drawn for alkene products?

Answer 55

alkenes and subs should be drawn in plane because they are trigonal planar

Question 56

What is the most important factor in determining E1 or E2?

Answer 56

strength of base; E2 = strong base; E1 = weak base

Question 57

Can E1 reactions occur for methyl or 1º?

Answer 57

No, but E2 can

Question 58

What is the only reaction that methyl can undergo?

Answer 58

Sn2

Question 59

What reactions can 1º undergo?

Answer 59

Sn2, E2

Question 60

What reactions can 2º undergo?

Answer 60

Sn2, E2, Sn1, E1

Question 61

What reactions can 3º undergo?

Answer 61

E2, Sn1, E1

Question 62

Do 1º benzylic or allylic compounds undergo E2?

Answer 62

no, only Sn2 or Sn1 (or E1 if there is resonance)

Question 63

Do 3º benzylic or allylic compounds undergo Sn2?

Answer 63

no, only E2, Sn1 and E1

Question 64

When do E1 reactions go for 1º allylic and benzylic ompounds?

Answer 64

not for benzylic but if there is resonance in allylic